The present invention relates to a method and apparatus for compact thermal control for avionics pod cooling units. More particularly, the present invention relates to thermal control in which refrigerant is circulated adjacent to the pod in a closed loop system in which the electronic cold plate can also serve as an evaporator in a heat pump loop or a heat exchanger in a pumped coolant loop.
Today's typical fighter aircraft must fly both numerous missions and different types of missions. One way to provide such aircraft with varying capabilities is to attach a particular type of avionics (electronics) pod below the aircraft with mission specific electronics contained therein. In this way, attachment of particular pod configures the aircraft for that particular mission. In using a pod, however, there are size, weight, and electrical consumption constraints. In addition, little space is available for the required cooling of the electronics contained in the pod in order to prevent overheating of the electronics. Typically, the cooling system is crammed into the back of the pod because the antenna for the electronics typically must utilize the front of the pod.
A known way to cool electronics pods is via direct or indirect convective heat transfer with the air surrounding the pod. Direct or indirect cooling of all the electronics in a typical pod merely with the air passing by the pod is, however, not practical for most electronics which typically must be kept at or around ambient air temperature, but during supersonic flight at low altitude the ambient air temperature can easily exceed 100.degree. C. (212.degree. F.).
Cooling on the ground, e.g., preflight check-out, is also required for periods ranging from thirty minutes to several hours. A simple convection air cooling method would therefore require a ground cooling cart to pre-cool the air on the ground. Direct convection cooling would occur by diverting some of the air flow which is passing by the pod and directing this air against the electronics being cooled. For example, FIG. 1 shows a conventional arrangement in which an electronics pod is mounted on a finned heat exchanger.
Indirect convection cooling can occur by using a conventional heat exchanger, i.e. a radiator, in the air flow and pump a coolant around a loop from this heat exchanger to heat exchangers, i.e. cold plate(s), located on the electronics as seen in FIG. 2. The heat transfer coolant in the indirect cooling method can be a single phase coolant. That is, the coolant always remains a liquid or vapor during the cooling process and thus does not change phase. Alternatively, the coolant can be a two-phase coolant which boils or evaporates at the electronic cold plates and condenses in the radiator.
Simple convection heat transfer methods have been used when the operation of the device is not highly temperature sensitive. Higher operating temperatures and operating temperature swings present in such a system do, however, affect the life of any electronic device. The thermal expansion and contraction of the interface connectors has also been shown to decrease system reliability.
Known convection cooling is practical for cooling the radar antenna and similar non-temperature-sensitive electronics. An antenna is not 100% efficient, and therefore some of the energy sent to the antenna is lost as a result of the antenna inefficiency. This energy is dissipated as heat. This waste heat must be dissipated, but such antennas as are typically not very temperature sensitive and convection cooling is adequate.
Limited temperature control to prevent the opposite problem, namely the electronics from becoming too cold, can be accomplished with the convection cooling methods by shutting off the convective air flow in the direct cooling configurations or by turning the pump off in the indirect convection cooling methods.
Convection cooling is only useful when the ambient temperature or the heat sink is at a lower temperature than the desired electronics temperature. One way to extend the operating range, is to utilize a water boiler. As the radiator temperature reaches the boiling point of water, water can be boiled off, to provide cooling at approximately the boiling point of water. The boiling point of water decreases as the pressure decreases, and so the boiling point temperature drops as the altitude of the airplane increases. However, the major aerodynamic heating of the ambient air surrounding the aircraft occurs at low altitude supersonic flight conditions and at these low altitudes the boiling point of the water is essentially 100.degree. C. A Venturi-type of nozzle can utilize the speed of the air to drop the pressure in the water boiler, thereby lowering the boiling temperature, but the quantity of water needed is too substantial to be practical except for short duration peak load reduction applications. Again, because most electronics must be kept cooler than the sea level boiling point of water, this water boiler method has been considered in the past to have limited applications. Other lower temperature fluids can also be boiled off but these fluids typically pose environmental and logistical supply problems.
Because the simple conduction cooling methods are not adequate for a typical electronics pod, active cooling systems have also been implemented. The most compact and energy efficient active cooling method is the vapor-compression air conditioning system. This known system in its basic form, as shown in FIG. 3, uses a well known evaporator, condenser, compressor and expansion valve. To date, these systems have been added to the existing convection cooling systems.
The system depicted in FIG. 3 also shows a filter-dryer. The filter-dryer is used to prevent clogging of the expansion device. An expansion device comprising a Thermal Expansion Valve (TXV) is used. However, any type of commercially available expansion device such as a capillary tube, electronic expansion valve, low-side or high-side float control, expansion orifice or the like could be substituted for the TXV.
One published previous investigation that addressed the system design for the thermal management of pod electronics was a system developed by Sundstrand. The environmental control units for navigation and targeting pods were developed to dissipate 2.6 kW of heat during flight and 2.0 kW of heat on ground. This study used a R-114 vapor compression cycle employing a semi-hermetic motor-driven rolling-piston rotary compressor, and a conventional thermal expansion valve. The condenser was cooled by a fan on the ground and by ram air during flight. The coolant temperature was maintained by switching between four modes of operation described as (i) heating by way of a heater located in pumped coolant loop, (ii) neutral with no cooling or heating, (iii) bypass with indirect convection cooling to ambient air, and (iv) cooling by using the vapor compression system. This system is representative of known vapor-compression systems which are added to existing systems, in which the complexity of the overall system is undesirably increased.
FIG. 4 is a schematic diagram of a typical thermal control system using a vapor-compression cooling system, indirect convection cooling loop, and a heating loop. In such systems, the coolant which is pumped through the electronics cold plate is a single-phase coolant and is cooled by direct heat transfer with the ram air, or by contact with the evaporator of the vapor-compression air conditioner. This same fluid is heated by the electric heater. The refrigerant of the vapor compression cooling system does not pass directly through the cold plates.
It is an object of the present invention to provide an improved method of providing combined and compact thermal control. This object has been achieved by providing a compact thermal control system in which the refrigerant is circulated through cooling channels adjacent to the pod electronics such that refrigerant passes directly through cooling passages in the electronics cold plate. This arrangement thus forms the evaporator of a heat pump loop or the heat exchanger of a pumped coolant loop, thereby resulting in a more compact arrangement.
A main advantage of the present invention is that the same refrigerant working fluid and many of the same components are used for both convection cooling and compressor-assisted cooling, thereby reducing the total number of components needed. Heat is directly transmitted from the pod electronics to the refrigerant. Depending on the ambient heat rejection capability (e.g. air temperature, air speed), the fluid can be circulated by a compressor forming a heat pump or by a pump forming a pumped single-phase or two-phase loop. Generally speaking, at lower ambient temperatures, the pump is operational while, at less favorable heat rejection conditions the compressor operates thereby elevating the condenser pressure and temperature to cause heat rejection.